Sputtertarget

ABSTRACT

This invention consits of a sputtering target used in carrying out a PVD coating process where the sputtering target is sputtered by bombardment with gas atoms and a layer consisting of several metallic elements is deposited onto a substrate, the sputtering target being a plate ( 2 ) made of a metal used for building up said layer, and with the other metals used for building up the layer bieng present at least partially in the form of plugs ( 1 ), which are inserted in holes in the plate ( 2 ), the shape of the free surfaces of the plugs ( 1 ) being selected in such a way that the sputtering rate for each metal used in sputtering process can be set according to the desired composition of the layer being applied.

[0001] The invention relates to a sputtering target for carrying out aPVD coating process and a PVD coating process where the sputteringtarget is sputtered by bombardment with gas atoms and/or ions and alayer consisting of several metallic elements is deposited on thesubstrate, the sputtering target being a plate made of one of the metalsused for building up the said layer, with the remaining metals used forbuilding up the layer at least partly in the form of plugs, which areinserted into holes in the plate.

[0002] A target structured in this way is generally referred to as a“mechanical sputtering target”. Before a PVD coating process is carriedout, the sputtering target is fitted to a cathode on the equipment usedfor carrying out the process and is electrically connected to thiscathode. For this reason, the sputtering target possesses the sameelectrical potential as the cathode attached to this equipment.

[0003] As a result of bombardment with gas ions, atoms from the plateitself and the plugs in the plate are removed from the surface of thesputtering target as dust to enter the gas phase. The sputtered-offmetal atoms are separated from the gas phase and deposited on to thesurface of a substrate, such as an indexable insert or a twist drill. Inthis way, the different types of metal atoms are applied to thesubstrate in a certain ratio, thus guaranteeing the stoichiometry of thelayer being applied.

[0004] If, for example, a Ti/Al sputtering target is used and a normalTiAlN coating layer is to be applied to the substrate, the two metalscan be present in a ratio of 1:1, the overall layer being composed of 25at % Ti, 25 at % Al and 50 at % N.

[0005] With a sputtering target of this type, the plate consists oftitanium, while the plugs are of aluminium. In the case of previouslyknown mechanical sputtering targets, the surface to be treated isflat—i.e. the free surfaces of the plugs lie flush with the surface ofthe plate itself.

[0006] The disadvantage of a known sputtering target of this type isthat, at the beginning of the cathode sputtering process, the ratio(e.g. between sputtered-off titanium and aluminum atoms) required forbuilding up the desired layer is not achieved. The sputtering rates donot reach the required ratio until after a lengthy starting period(often of several hours' duration) for the sputtering target. This meansthat the material that is already being sputtered off the plate andplugs during the run-in phase is wasted, as it cannot be used in theactual coating process.

[0007] In the case—for example—of a mechanical sputtering target with atitanium plate and aluminium plugs, it can be noticed during bombardmentof the target that the sputtering rate for aluminium is considerablyhigher than that of titanium. For this reason, the plugs wear down at afaster rate than the plate itself. The result of this is that a largepart of the plate material remains intact after the plugs have beencompletely worn away.

[0008] DE-A-2914618 contains a description of a process for theapplication of a sliding or friction surface to a substrate by means ofcathode sputtering. The targets used in this process are objects with acircular cross-section, in which the materials used to form the slidingsurface are combined, the object being made of a matrix material withholes containing plugs made of other materials.

[0009] The article “Oxidation behaviour of nanocrystalline Fe—Ni—Cr—Alalloy coatings” by Z. Liu, W. Gao and Y. He, published in the December1999 issue of “Materials Science and Technology”, Vol. 15, page 1447 ffdescribes a coating process involving Fe—Ni—Cr—Al alloys, which usesmagnetron—sputtering technology. The article cites the use of a targetwith a diameter of 150 mm and a thickness of 6 mm, the eight plugsinserted into the target being of pure aluminium and of 8 mm indiameter, in order to create Fe—Ni—Cr—Al alloy coatings.

[0010] DE-A465699 describes a process for the coating of a substrate bymeans of cathode sputtering, along with a cathode (target) used for thispurpose. In order to create a regular alloy coating consisting of two ormore materials on the substrate, the target used consists of a metalplate made of one metal, in which pins of a second metal are insertedinto holes. These pins can be used for fixing plates of the secondmaterial to the surface. It is indicated that correct adjustment of theratio of the free surfaces can be used to create the desired coating.

[0011] It is the object of the invention to obtain a sputtering targetthat is more viable and cost-effective when carrying out a PVD coatingprocess with cathode sputtering.

[0012] This object is achieved, in the case of the item initiallyreferred to as the sputtering target, by selecting the shape of the freesurface of the plugs in such a way that the sputtering rates for eachmetal required for the desired coating composition are achieved whensputtering the target.

[0013] The teaching of the invention consequently involves using theshaping of the free surfaces of the plugs to take into account thedifferent sputtering rates of the metals of the plate and the plugs.Consequently, if the sputtering rate for the plug material is greaterthan the sputtering rate for the plate material, the free surface of theplugs runs inwards relative to the surface of the plate. In this way,the drill holes in the plate act as a screen for the gas atoms/ions asthey hit or leave the sputtering target. For this reason, the actualsputtering rate for the plug metal is reduced to such an extent that thesputtering rates required for the desired substrate-coating compositionare reached both for the plate metal and the plug metal.

[0014] If, however, the sputtering rate for the plug material is smallerthan the sputtering rate for the plate material, the free surface of theplugs preferably runs outwardly relative to the surface of the plate.

[0015] In comparison to prior sputtering targets, the sputtering targetthat is the subject of this invention requires no run-in time—thusresulting in a more cost-effective use of sputtering-target material.

[0016] The plate can be made of a single metal. It is however alsopossible to use a plate made of alloy material. The material used forthe plugs can vary from plug to plug. The selection of materials dependsexclusively on the composition of the layer being applied to thesubstrate.

[0017] Typical combinations of plate and plug materials includetitanium/aluminium, titanium/zirconium and titanium/carbon, the lastmaterial listed being plug material. When carbon is used as a plugmaterial, the plug has a lower sputtering rate than the titanium used tomake the base-plate. In this case, the plug would protrude from thesurface of the plate by the corresponding amount.

[0018] It is normally preferable that the actual desired sputtering rateof the plug material is set by selecting the amount by which the plugsinks into or protrudes out of the holes in the plate.

[0019] It is not absolutely vital that the free surface of each plugshould lie exactly flush with the surface of the plate. Experiments havenevertheless shown that it is preferable for the free surfaces of theplugs to form a single surface with that of the plate, the free surfacesof the plugs being curved inwards or outwards relative to the plate. Inother words, the free surfaces of the plugs should be linked to thesurface of the plate. The curvature of the free surface of the plugscorresponds, as already described above, to the sputtering-rate ratio ofthe plate material to the plug material.

[0020] The holes should preferably be circular in cross-section, whilethe radius of curvature of the surfaces of the plugs should maintain aratio, with respect to the diameter of the hole, of 1.5:1 to 2.5:1.

[0021] In the case of a plug diameter of 15 mm for an aluminium plug ina titanium plate, the best coating conditions are obtained if the radiusof curvature of the free surface of the aluminium plugs has been keptwithin a range of 25 to 35 mm.

[0022] If the plug has to be deformed to obtain the desired amount ofsurface curvature, it is preferred that the plug material is softer thanthe material of which the base-plate is made.

[0023] The base-plate (which can be made of such materials as titanium,chrome, stainless steel, vanadium, nickel, zirconium, hafnium, tantalumor carbon) is attached—as known in the art—to a cooling plate, which isnormally made of copper.

[0024] The cost-effective use of the sputtering target is also increasedby making the plugs protrude from the main plate and into holes in thecooling plate (in cases where the plug material wears down faster thanthe plate material). In the case of a titanium plate with aluminiumplugs, the plate can be 5 mm thick, while the plugs are 7 mm long, thusleaving 2 mm of aluminium plug free to protrude into the cooling plate.

[0025] After using a sputtering target of this type to apply layers tosubstrates, an optimum amount of titanium is consumed from the plate,despite the higher sputtering rate of aluminium—with 1 mm (for example)of aluminium plug still remaining in the cooling plate. It is notpossible to consume all the titanium in the plate, as a surroundingtrough is formed, during the coating process, between the central areaof the sputtering target and its edge (magnetron target), which is dueto the sputtering process in the sputtering target. Once the depth ofthe trough coincides with the thickness of the base-plate, thesputtering target is no longer usable.

[0026] Special attention is drawn to the fact that the method of makingthe plugs protrude into the cooling plate supposes advantages in itself,quite apart from the items described above with respect to theinvention. For this reason, the configuration of a sputtering target ofthe type described initially (with plugs protruding into the coolingplate) is regarded as a separate solution of the object of thisinvention.

[0027] The protrusion of the plugs into the cooling plate offers theadditional advantage of more effective cooling of the plug material. Inparticular, this layout also prevents plate material getting between thecooling plate and plugs while the cathode sputtering process is beingcarried out. This is of special advantage if the plate materialpossesses low heat conductivity (as with titanium, for example).

[0028] Different configurations of the invention are further-explainedbelow with the aid of drawings. Items illustrated:

[0029]FIG. 1 Top view of a sputtering target

[0030]FIG. 2 Partial cross-sectional view of a sputtering target withplugs (which have a curved free surface) protruding into a coolingplate.

[0031] A typical sputtering target consists of a titanium base-plate,with holes for pressing in the aluminium plugs. The sputtering target isabout 90 mm wide and can be between 100 mm and 800 mm long. Otherdimensions are available as required.

[0032] The number of aluminium plugs used depends on the proportion ofaluminium required in the chemical composition of the coating to be usedfor treating the substrate. The proportion of aluminium in thesputtering target can be between 1 and 50 at %.

[0033] The plugs typically measure 15 mm in diameter, while base-platethickness can range from 5 mm to 8 mm.

[0034]FIG. 1 shows a Ti/Al sputtering target with titani as base-platematerial and plugs made of aluminium. The aluminium plugs (1) are evenlydistributed across the surface of the titanium base-plate (2).

[0035]FIG. 2 shows the section of the sputtering target that is markedwith an “X” in FIG. 1. As the cross-sectional view shows, the aluminiumplugs (1) have a concave free surface relative to the base-plate (2).The free surface of the plugs (1) lies flush with the surface of thebase-plate (2). The plugs (1) protrude into holes in the cooling plate(3). The amount of protrusion ensures optimum use of the sputteringtarget during the cathode sputtering process—despite the differentsputtering rates of titanium and aluminium. The purpose of the coolingplate (3), which is made of copper, is to dissipate the heatproduced—during the sputtering of the sputtering target—on the side ofthe target that is being subjected to the sputtering process.

[0036] The skilled person selects a concave or convex curvature for theplugs used in each combination of materials for the base-plate (2) andplugs (1), in order to obtain the desired sputtering rate.

[0037] In the example of a Ti/Al sputtering target, the aluminium plugsprotrude by at least 1.5 mm into the cooling plate (3), which is between3 mm and 6 mm thick. This shape of the inserted plugs is selected herebecause—with regular bombardment of gas atoms and/or ions—the plugmaterial (in this case: aluminium) possesses a high sputtering rate thanthe plate material (in this case: titanium). The sunken shape of thesurface means that fewer gas atoms and/or ions actually hit the plugmaterial, with the ring-shaped section of surrounding plate-materialacting as a screen.

[0038] The skilled person is familiar with the sputtering rates of thevarious materials used. With these rates in hand, the required shape ofthe surface for each combination of plate and plug materials can bedetermined.

1. Sputtering target for carrying out a PVD coating process where thesputtering target is sputtered by bombardment with gas atoms and/or ionsand a layer consisting of several metallic elements is deposited on thesubstrate, the sputtering target being a plate (2) made of one of themetals used for building up the said layer, with the remaining metalsused for building up the layer at least partly in the form of plugs (1),which are inserted into holes in the plate, characterised in that theform of the free surface of the plugs (1) is selected in such a way thatsputtering rates are achieved for the sputtering of each target metal,as required for the desired composition of each coating layer. 2.Sputtering rate according to claim 1, characterised in that thesputtering rate of the plug material is set by the amount by which theplug (1) sinks into or protrudes out of the holes in the plate (2). 3.Sputtering target according to claims 1 or 2, characterised in that thefree surface of the plugs (1) and that of the plate (2) form a singlesurface, with the free surfaces of the plugs (1) being curved inwards oroutwards relative to the plate (2).
 4. Sputtering target according toone of the preceeding claims, where the free surface of the plugs (1)runs inwardly relative to the surface of the plate (2), if thesputtering rate of the plug material is higher than the sputtering rateof the plate material, and where the free surface of plugs (1) runsoutwardly relative to the surface of the plate (2), if the sputteringrate of the plug material is lower than the sputtering rate of the platematerial.
 5. Sputtering target according to one of the preceeding claims, characterised in that the holes are of circular cross-section and thatthe radius of curvature of the free surfaces of the plugs (1) relativeto the hole diameter lies within a ratio of 1.5:1 to 2.5 :1. 6.Sputtering target according to claim 5, characterised in that the plate(2) is made of titanium and the plugs (1) are made of aluminium. 7.Sputtering target according to claim 6, characterised in that the holediameter is 15 mm and the radius of curvature of the free surfaces ofthe aluminium plugs (1) lies within a range of 25 mm to 35 mm, the freesurfaces of the plugs (1) being curved inwards relative to the plate. 8.Sputtering target for carrying out a PVD coating process where thesputtering target is sputtered by bombardment with gas atoms and/or ionsand a layer consisting of several metallic elements is deposited on thesubstrate, the sputtering target being a plate (2) made of one of themetals used for building up the said layer, with the remaining metalsused for building up the layer at least partly in the form of plugs (1),which are inserted into holes in the plate, the main plate beingattached to a cooling plate (3) by means of the plugs, characterised inthat the plugs (1) in the plate (2) protrude out of the holes and intothe cooling plate (3).
 9. Sputtering target according to claim 8,characterised in that the plugs (1) in the main plate (2) protrude by atleast 1.5 mm into the holes in the cooling plate (3).
 10. PVD coatingprocedure, during which a sputtering target is sputtered by bombardmentwith gas atoms and/or ions, and a layer consisting of several metallicelements is deposited onto a substrate, the sputtering target being aplate (2) made of a metal suitable for the build-up of the said layer,the other metals being present at least partly in the form of plugs (1)inserted into holes in the plate (2), characterised in that a desiredlayer-composition is selected and plugs (1) are used, with a freesurface shaped in such a way that the sputtering rate for each metal canbe set corresponding to the desired composition of the layer.